Tickle, I'm not so convinced. A search doesn't bring up much, but if it was detecting the humidity levels then it was probably some kind of saturation indicator.
Do you remember the colors it went mjacobs?
The information I can find suggests that one of the best chemicals for the job would be
Cobalt (II) Chloride.
This turns from blue (CoCl2), to purple when it absorbes 2 water molecules (CoCl2.2H2O) to pink when it has absorbed 6 water molecules (CoCl2.6H2O).
This reaction is reversible. So it will turn back to CoCl2 when the water is driven off by dry weather.
Does this tie in with how you remember it changing color? I have to admit I'm too young to have seen these devices.
Further reading:
Cobalt Chloride Paper. This is used as an indicator for water.
Please keep in mind that if this sounds like the stuff then it is now labelled as Toxic, Harmful to the environment and "Probably carcinogenic". Back in the 50's it was probably enough to label it as "WARNING - TOXIC" but nowadays it's a little more compicated. This is not stuff you want to let kids near, or to play about with if you're not sure of what you're doing.
It'll probably cost a bit to dispose of correctly too.